A typical touch screen employs a sheet of glass with a conductive coating such as indium tin oxide with four terminal connections, one at each corner. The touch screen may also be a capacitive or resistive touch screen with a pattern of electrodes made of conductive material. A finger, stylus, or conductive top sheet can draw or inject current at the point of contact. The current can then distribute to the touch panel terminals in a proportionate manner relative to the location of the point of contact. The accuracy of the touch screen thus depends on how well the division of current among the terminals represents the contact location. To help correlate point of contact signals with correct position data, touch screens are calibrated. Calibration usually takes place during manufacturing, and in some cases recalibration can be performed by an end user, for example. Calibration can also include diagnostic functions whereby uncorrectable errors can be found and a non-functioning status can be indicated.